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f : A → B is an injection ⇔ a₁ , a₂ ∈ A and a₁ ≠ a₂ implies that f(a₁ ) ≠f( a ₂ )

⇔a₁ , a₂ ∈ A and f(a₁ ) = f(a₂ ) = f(a₁ ) implies a₁ = a₂

surjection : A function f : A→B is called a surjection if the range of f is equal to the co-domain of f.A surjection is also called onto.

f : A → B is a surjection ⇔range f = f(A) = B ( c0- domain)

⇔ B = { f(a) / a∈ A}

⇔ for every b∈B there exists atleast one a∈A such that f(a) =b